In optical networks, signals may be transmitted at various wavelengths, with each wavelength corresponding to a transmission channel. Optical links may connect network nodes so that signals may be transmitted throughout the optical network. An optical route may use a series of network nodes and optical links to connect a source of an optical transmission with a destination for the optical transmission.
Reconfigurable optical add-drop multiplexers (ROADMs) have seen increasing usage as optical nodes. An optical network that is implemented using ROADMs can be reconfigured without physical modification of components of the ROADMs, which improves flexibility and peak bandwidth of the optical network. A ROADM may include a wavelength selective switch (WSS), which includes a switching engine that routes optical signals to particular output ports based on wavelengths of the optical signal.
As traffic demand increases in optical fiber, there is an increasing usage of higher-order coherent modulation formats such as DP-8QAM and DP-16QAM, which may drive commensurate improvements in optical signal-to-noise ratio (OSNR). For these spectral efficiency improvements to be possible, OSNR degradations should be reduced or avoided. One way to combine/split sub-carriers of a super-channel into/from a WSS ROADM is to use an external passive 1×N coupler (or multicast switch) prior to connection to a WSS. In such a case, the deployed WSS ‘channel’ slot may have a bandwidth approximately equal to the sum of the baud rates of the sub-carriers. Such a 1×N coupler/splitter may introduce significant insertion loss at both transmit and receive, which drives the use of additional optical amplifiers in the path and, thus, degrades the achievable OSNR and reach. Where amplifiers are used to boost sub-carrier output power prior to multiplexing, broadband ASE (Amplified Spontaneous Emission) noise may accumulate in a passive coupler which could degrade OSNR by 10×log(N), where N is the number of ports combined in the coupler.
It would be advantageous to be able to create spectrally efficient super-channels in an optical network without suffering significant insertion loss. It would be advantageous to be able to create spectrally efficient super-channels in a WSS from optical channels provided on different input ports without physically reconnecting inputs to the WSS. It would be advantageous to be able to de-multiplex spectrally efficient super-channels in an optical network without suffering significant insertion loss.